The present invention relates to a spring steel used for high strength springs employed for automobiles, other industrial machines, and the like.
As automobiles having a high performance have come to be produced, the springs used therein must be very strong, and a high strength steel having a tensile strength exceeding 150 kgf/mm2 after heat treatment has been used for the springs. A steel having a tensile strength exceeding 200 kgf/mm2 has also been used in recent years. Japanese Unexamined Patent Publication (Kokai) No. 57-32353 discloses a procedure wherein fine carbides which are brought into solid solution by quench-hardening and which are precipitated by tempering are formed in the steel by adding elements such as V, Nb and Mo, and the fine carbides limit the movement of dislocations and improve the resistance to setting.
However, it is important that a steel for springs has such a fracture property that the steel can withstand the harsh environment where the springs are used. In particular, it is well known that when the strength of the steel is increased, the impact toughness and the ductility thereof lower. The impact toughness of the steel disclosed in Japanese Unexamined Patent Publication (Kokai) No. 57-32353 is from 2.2 to 2.8 kgf-m/cm2 as measured using a JIS No. 3 test piece. Therefore, it can be concluded that the steel can never have a sufficiently high toughness.
Fatigue property in a corrosive environment must be also considered from the view points of the pit formation and hydrogen absorption due to corrosion in addition to the usual factors facilitating fatigue in a dry environment. It is generally recognized that, similarly to impact toughness and elongation, the corrosion fatigue resistance is lowered with an increase in the steel strength and that no practically acceptable steels could be obtained if a conventional steel is strengthened by heat treatment alone.
An object of the present invention is to provide a steel material for springs having a high strength and a high toughness after heat treatment.
The present inventors have developed a steel having a sufficient ductility and a sufficient impact toughness, even when the steel is made to have a high strength, by refining austenite grains with precipitates which have never been observed in conventional spring steels, and extremely decreasing the impurities at austenite grain boundaries which tend to promote fracture.
The object as mentioned above can be attained by the present invention described below.
A first aspect of the present invention provides a high toughness spring steel comprising, based on mass, 0.35 to 0.85% of C, 0.9 to 2.5% of Si, 0.1 to 1.2% of Mn, 0.1 to 2.0% of Cr, 0.005 to 0.07% of Ti, 0.001 to 0.007% of N, the Ti content being greater than four times the N content in terms of percent by mass, P and S with restrictive contents of less than 0.020% and less than 0.020%, respectively, and the balance of Fe and unavoidable impurities.
A second aspect of the present invention provides a high toughness spring steel comprising, based on mass, 0.35 to 0.85% of C, 0.9 to 2.5% of Si, 0.1 to 1.2% of Mn, 0.1 to 2.0% of Cr, 0.005 to 0.07% of Ti, 0.0005 to 0.0060% of B, 0.001 to 0.007% of N, the Ti content being greater than four times the N content in terms of percent by mass, P and S in restrictive contents of less than 0.020% and less than 0.020%, respectively, and the balance of Fe and unavoidable impurities.
A third aspect of the present invention provides a high toughness spring steel further comprising, based on mass, one or two kinds of the following elements with the following contents: 0.05 to 0.5% of V and 0.01 to 0.10% of Nb, in addition to the elements defined in the first or the second aspect of the present invention.
A fourth aspect of the present invention provides a high toughness spring steel further comprising, based on mass, one or two kinds of the following elements with the following contents: 0.05 to 1.0% of Ni and 0.05 to 1.0% of Mo, in addition to the elements defined in the first or the second aspect of the present invention.
A fifth aspect of the present invention provides a high toughness spring steel further comprising, based on mass, one or two kinds of the following elements with the following contents: 0.05 to 0.5% of V and 0.01 to 0.10% of Nb, and one or two kinds of the following elements with the following contents: 0.05 to 1.0% of Ni and 0.05 to 1.0% of Mo, in addition to the elements defined in the first or the second aspect of the present invention.
A sixth aspect of the present invention provides a high toughness spring steel further comprising, based on mass, 0.05 to 0.3% of Cu, in addition to the elements defined in the first or the second aspect of the present invention.
A seventh aspect of the present invention provides a high toughness spring steel further comprising, based on mass, 0.05 to 0.5% of Cu and 0.05 to 1.0% of Ni, the Cu content being less than the Ni content in terms of percent by mass provided that the Cu content is greater than 0.3%, in addition to the elements defined in the first or the second aspect of the present invention.
An eighth aspect of the present invention provides a high toughness spring steel further comprising, based on mass, one or two kinds of the following elements with the following contents: 0.05 to 0.5% of V and 0.01 to 0.10% of Nb, in addition to the elements defined in the sixth or the seventh aspect of the present invention.
A ninth aspect of the present invention provides a high toughness spring steel further comprising, based on mass, 0.05 to 1.0% of Mo, in addition to the elements defined in the sixth or the seventh aspect of the present invention.
A tenth aspect of the present invention provides a high toughness spring steel further comprising, based on mass, one or two kinds of the following elements with the following contents: 0.05 to 0.5% of V and 0.01 to 0.10% of Nb, and 0.05 to 1.0% of Mo, in addition to the elements defined in the sixth or the seventh aspect of the present invention.
In a further aspect, the present inventors found that addition of Mg, La and/or Ce refines and disperses coarse MnS or other sulfide and oxide inclusions to suppress formation of corrosion pits which provide the starting points for fatigue fracture, thereby ensuring a good corrosion fatigue property of springs.
The present inventors also found that addition of B mitigates the amount of P segregated on the prior austenite grain boundaries to further improve impact toughness and elongation in comparison with conventional steels in which the gross P amount was simply reduced.
According to the further aspect, there is provided a high toughness spring steel comprising, based on mass, 0.45 to 0.85% C, 0.9 to 2.5% Si, 0.1 to 1.2% Mn, 0.1 to 2.0% Cr, 0.005 to 0.07% Ti, 0.001 to 0.007 N, the Ti content being greater than four times the N content, 0.0005 to 0.0060% B, at least one of 0.0005 to 0.01% Mg, 0.0005 to 0.01% La and 0.0005 to 0.01% Ce, P and S with respective contents of less than 0.020% and 0.020%, and the balance of Fe and unavoidable impurities, and percent area of oxides and sulfides being not more than 0.1%.
The upper limits of the Mg, La and Ce contents are preferably 0.003%, 0.007% and 0.007%, respectively.
In addition to the above-mentioned basic constituents, the steel preferably further comprises one or two of 0.05 to 0.5% V and 0.01 to 0.10% Nb and/or one or two of 0.05 to 1.0% Ni and 0.05 to 1.0% Mo.
In addition to the basic constituents, the steel preferably further comprises either 0.05 to 0.3% Cu, or 0.05 to 0.5% Cu and 0.05 to 1.0% Ni with the Cu content being less than the Ni content provided that the Cu content is greater than 0.3%.
The above-mentioned Cu containing steel preferably further comprises one or two of 0.05 to 0.5% V and 0.01 to 0.10% Nb, and/or 0.05 to 1.0% Mo.